Photoconductive layer comprising a mixed crystals of lead monoxide and tin oxide

ABSTRACT

In a photoconductive device, more particularly a camera tube of the Vidicon type, comprising a photoconductive layer on the basis of lead monoxide this layer consists at least partly of mixed crystals of lead monoxide and tin monoxide. The content of tin monoxide in the mixed crystals preferably lies between 0.1 and 10 mol percent.

United States Patent [72] Inventors Wim Kwestroo;

Johannes Van Den Broek; Adriaan Netten, all of Emmasingel, Eindhoven, Netherlands 211 App]. No. 33,548

[22] Filed May I, 1970 [45] Patented Oct. 5, 1971 [7 3] Assignee U.S. Philips Corporation New York, NY.

[32] Priority May 27, 1969 [33] Netherlands [54] PHOTOCONDUCTIVE LAYER COMPRISING A MIXED CRYSTALS 0F LEAD MONOXIDE AND TIN OXIDE 5 Claims, 2 Drawing Figs.

[52] US. Cl 313/65 A, 313/94, 317/235 N, 317/235 AQ, 317/238 51 int. Cl .l ..ll0lj 31/38,

Primary Examiner-Robert Segal AttorneyFrank R. Trifari ABSTRACT: In a photoconductive device, more particularly a camera tube of the Vidicon type, comprising a photoconductive layer on the basis of lead monoxide this layer consists at least partly of mixed crystals of lead monoxide and tin monoxide. The content of tin monoxide in the mixed crystals preferably lies between 0.1 and 10 mol percent.

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JOHANNES v. don BROEK, ADRIAAN NgTTEN PHOTOCONDUCTIV E LAYER COMPRISING A MIXED CRYSTALS OF LEAD MONOXIDE AND TIN OXIDE The invention relates to a photoconductive device, particularly a pickup tube of the Vidicon type having a photoconductive layer on the basis of lead monoxide applied to a support and being provided with at least one electrode and to methods of manufacturing such a device.

In known camera tubes of the aforesaid type, marketed under the trade name of Plumbicon," the photoconductive layer is formed by a more or less porous layer of tetragonal (red) lead monoxide in which sulfur, selenium or tellurium may be absorbed for enhancing the red sensitivity. The absorption of one or more of these elements increasing the red sensitivity is obtained by exposing the lead monoxide during or after the vapor deposition of the photoconductive layer to a gaseous hydrogen compound of one or more of these elements i.e. sulfur-, seleniumor tellurium-hydride or mixtures thereof. This method of obtaining an increased red sensitivity of the photoconductive layer requires a comparatively accurate dosing of said hydrogen compound under accurately reproducible conditions in order to obtain in a reproducible manner photoconductive layers having on the one hand the desired red sensitivity and on the other hand not exhibiting the dark current which is particularly undesirable for use in camera tubes. Moreover, the disadvantage is involved here that the resultant red sensitivity occurs in the form of a comparatively low tail of the spectral sensitivity curve of the lead monoxide contrary to what is desired, i.e. in the form of a displacement of the curve portion concerned towards an absorption limit of longer waves.

The invention is based on the discovery that the desired red sensitivity of a photoconductive layer on the basis of lead monoxide can be obtained in a less critical manner by incorporating, especially in the fonn of a mixed crystal with lead monoxide, a metal oxide having a smaller energy gap that the lead monoxide. It has been found that tin monoxide is a par ticularly suitable material for this purpose. lt should be noted that whereas with tetragonal lead monoxide the energy gap is about 1.9 ev.'this is about 0.6 ev. with tin monoxide. ln accordance with this invention a photoconductive device of the kind set forth is characterized in that the photoconductive layer comprises mixed crystals of lead monoxide and tin monoxide. In an advantageous embodiment the tin monoxide content in the mixed crystals lies between 0.1 and 10 mol percent. A tin monoxide content of x mol percent means that in the mixed crystal the ratio between the numbers of tin atoms and lead atoms corresponds to 100-x. The photoconductive layer may consist substantially completely of said mixed crystals, particularly when the photoconductive device is formed by a camera tube for converting a radiation image captured by the photoconductive layer into electric signals, which tube comprises a photoconductive layer of a thickness lying between 5 and about 30 p. applied to a transparent support provided with an electric transparent electrode and an electron gun opposite the photoconductive layer for producing an electron'beam scanning the free surface of the photoconductive layer remote from the support, it being advantageous for avoiding an excessively high dark current to form the photoconductive layer portion adjacent the support substantially from mixed crystals of lead monoxide and tin monoxide and the layer portion including the free surface to be scanned by the electron beam substantially onlyfrom tetragonal lead monoxide.

According to a further aspect of the invention at least part of the photoconductive layer is formed by the vapor deposition of a powdery material of said mixed crystals provided in a crucible arranged opposite the support in an oxygen-containing atmosphere of a pressure of about torr.

A further method of manufacturing a photoconductive device in accordance with the invention is characterized in that at least part of the photoconductive layer is obtained'by the vapor deposition by way of vaporizing a mixture of'leadmonoxide powder and tin-monoxide powder on the support in an oxygen-containing atmosphere of a pressure of about 10' torr. According to the invention a photoconductive device of the kind set forth may also be obtained by vapor deposition of at least part of the photoconductive layer through the simultaneous evaporation of lead monoxide and tin from different crucibles in an oxygen-containing atmosphere of a pressure of about 10 torr.

The invention will be described with reference to the drawing, which shows an embodiment and variants thereof.

In the drawing FIG. 1 is a schematic sectional view of one embodiment of the photoconductive device according to the invention formed by a camera tube and FIG. 2 is a graph of the relationship between the energy gap and the tin monoxide content of mixed crystals of tetragonal lead monoxide and tin monoxide.

The camera tube shown in a longitudinal sectional view in FIG. 1, comprises an elongated, cylindrical vacuum bulb l of glass, closed at one end by a window 2 and at the other end by a glass base 3. This base, a central portion of which is formed by a sealed hollow exhaust tube 4, accommodates lead-in pins 5, which are electrically connected inside the bulb to various parts of an electrode system designated as a whole by 6. The electrode system 6 comprises a thermal cathode 8 to be heated by a filament wire 7, a Wehnelt cylinder 9 and a perforated anode 10, which is electrically connected to a cylindrical electrode 11. The electrode 11 is provided on the side of the window 2 with a gauze electrode 12.

The window 2 is provided on the inner side with a transparent, electrically good conducting signal electrode 13, for example, of conductive tin dioxide, connected to a current conductor 14, leading to the outside. The signal electrode 13 is provided by vapor deposition in a gas atmosphere containing oxygen and, if desired, also water vapor, with a photoconductive layer 15 of a thickness of about 15 to 25 [-L on the basis of lead monoxide (PbO). The layer 15, which forms the target plate of the tube, can be scanned on the free surface remote from the window 2 by means of an electron beam 16 emanating from the cathode 8 and focused and deflected on the target plate by means of the conventional deflection and focusing coils designated in common by 17, surrounding the tube.

With the aid of an optical system represented schematically by a single lens 19 an image is projected, when the tube is operating, onto the target plate 15 and converted into electric signals. The electric signals are produced in a conventional manner at the scan of the target plate by the electron beam 16 across a signal resistor 20, connected to the current conductor 14, while through said signal resistor the electrode 13 receives a bias voltage of 20 to 60 v. which is positive relative to the cathode 8 of the tube.

The target plate 15 consists wholly or partly of mixed crystals of lead monoxide and tin monoxide, the tin monoxide content in said crystals being less than 25 mol percent,

preferably lying between 0.1 and 10 mol percent. An advantageous value of the tin monoxide content is about 5 mol percent. It should be noted that the tin monoxide content of x mol percent means that of every metal atoms in the mixed crystal x atoms are tin atoms and 100-x atoms are lead atoms. If the target plate 15 does not wholly consist of mixed crystals of lead monoxide and tin monoxide, the remaining part thereof consists of crystals of tetragonal lead monoxide. ln this case the mixed crystals, if present, should be homogeneously distributed in each plane parallel to the signal electrode 13. This is achieved when in each part of the target plate the ratio between tetragonal lead monoxide and the mixed crystals is the same, which means that the target plate 15 is formed by a homogeneous layer. A further possibility resides in that the target plate 15 is formed by various partial layers lying one after the other in the direction transverse of the signal electrode 13,- in which the ratios between the quantities of tetragonal lead monoxide and mixed crystals are different. An advantageous embodiment is obtained when the target plate 15 is formed by a partial layeradjacent the signal electrode 13 consisting mainly of mixed crystals of lead monoxide and tin monoxide, in which the tin monoxide content is preferably about 5 mol percent and occupying about 90 percent or less of the overall thickness of the target plate 15, the remaining part of the target plate 15, forming the free surface to be scanned by the electron beam 16 being constituted by a partial layer.

In the graph of FIG. 2 for mixed crystals of lead monoxide and tin monoxide the energy gap E, in electron volts (ev.) is plotted as a function of the content of tin monoxide. The beginning of the curve, i.e. the part indicating of low tin monoxide content is shown on an enlarged scale in the righthand upper comer of this Figure. From FIG. 2 it will be apparent that the energy gap for pure lead monoxide is about 1.9 ev. and that with an increasing tin monoxide content the energy gap first drops comparatively rapidly and then slowly. In mixed crystals having a tin monoxide content of about 5 mol percent the energy gap is about 1.6 ev., with mol percent about 1.5 ev. and with 25 mol percent of tin monoxide the energy gap has dropped to about 1.3 ev. Since 1 ev. substantially corresponds to 1.24 z (wavelength), the absorption sides of the various mixed crystals and hence the sensitivity to optical radiation of longer wavelengths (i.e. red and infrared) can be readily determined. By including mixed crystals of lead monoxide and tin monoxide in the target plate the red sensitivity is extended in the range of longer wavelengths in accordance with the tin monoxide content of the mixed crystals as compared with a target plate consisting of lead monoxide only. Because with a decreasing energy gap of the material of the photoconductive layer 15 the dark current increases, it is desirable not to raise the tin monoxide content above mol percent, which may result in a sensitivity to radiation of a wavelength up to about 1 2. For a camera tube of the kind set forth, intended for conventional television purposes, a reasonable sensitivity to a wavelength of about 7000 A is usually largely sufficient. With such a tube the tin monoxide content will be chosen, in accordance with need, in the range from 0.1 to 10 mol percent and a very appropriate value lies about 5 mol percent. In the latter case the red sensitivity limit of the tube substantially corresponds with that of the Plum bicon tubes having an enhanced red sensitivity obtained by the action of hydrosulfide on the photoconductive target plate fonned by vapor-deposited tetragonal lead monoxide. The red sensitivity itself is, however higher than that of the said Plumbicon" tube.

The dark current can be restricted in the aforesaid manner by forming the target plate from two partial layers parallel to the signal electrode 13, i.e. a partial layer forming the free surface of tetragonal lead monoxide and a partial layer mainly of lead monoxide and tin monoxide mixed crystals adjacent the signal electrode. The first partial layer of lead monoxide may be obtained in known manner by the vapor deposition of pure lead monoxide in a gas atmosphere of a pressure of about 1X10 torr, consisting of a mixture of approximately equal quantities of water vapor and oxygen.

The target plate 15 or the part thereof consisting of mixed crystals of lead monoxide and tin monoxide can be obtained in various ways by vapor deposition in an oxygen atmosphere or an oxygen-containing atmosphere of a pressure of about 10 torr. A first method starts from powdery material of mixed crystals of lead monoxide and tin monoxide, in which the ratio between lead and tin corresponds with that in the mixed crystals in the final target plate or part thereof. This powdery material is transferred in known manner in the vapor phase from a crucible arranged opposite the signal electrode 13 to said signal electrode. Instead of a material consisting of mixed crystals, a mixture of powdery lead monoxide and powdery tin monoxide may be provided in the crucible, which mixture can then be evaporated. Since after the vapor deposition a small quantity of tin or tin oxide is left in the crucible, the ratio between tin and lead in the mixture should be slightly higher than is desired in the mixed crystals of the target plate 15 to be made. A third method of producing the target plate 15 or part thereof consists in the simultaneous evaporation of lead monoxide and tin from different crucibles arranged side by side. In this case the tin is heated at a higher temperature, for example, 1,000 to l,l00 C. than the lead monoxide (about 930 C.). The molecular percentage of tin monoxide in the vapor-deposited layer may be regulated by adjusting the temperature of the tin crucible; a higher temperature results in a higher percentage of tin monoxide.

The first method starts from mixed crystals of lead monoxide and tin monoxide, which can be obtained by the addition of an excess quantity of ammonia to a solution of tin perchlorate and lead acetate or tin perchlorate and lead perchlorate, in which solution the ratio between the molecular quantities of lead and tin is equal to that desired in the desired mixed crystals. The resultant precipitate is washed with very pure water and subsequently held at a higher temperature under water for some hours so that the precipitate is converted into mixed crystals of lead monoxide, and tin monoxide, the precipitate being than washed and dried. Two examples of such a production of mixed crystals are given below:

EXAMPLE 1 The process starts from 45 g. of substantially silicon-free tetragonal lead monoxide, which is dissolved in 75 ml. of perchloric acid of about 7 N. This solution is added to a solution of 5 g. of tetragonal tin monoxide in 75 ml. of perchloric acid of about 3 N. While stirring, an excess quantity of am,- monia is added to the joined solutions, a precipitate of leadtin-hydroxide being thus formed. This precipitate is washed with very pure water and subsequently held at a temperature of about 75 C. under water for some hours, for example, overnight. The initial lead-tin-hydroxide is thus converted into dark-brown mixed crystals of lead monoxide and tin monoxide having a tin monoxide content of 10 mol percent, the crystals being then washed and dried.

EXAMPLE 2 50 g. of tetragonal, substantially siliconfree lead monoxide is dissolved in ml. of perchloric acid of about 7 N. A second solution is made by dissolving 1.26 g. of tetragonal tin monoxide in 30 ml. of perchloric acid ofabout 3 N. The two solutions are added and while stirring about 1,700 ml. of concentrated ammonia is added. The resultant precipitate of lead-tinhydroxide is treated as described under example 1.

It should be noted that in this method of producing mixed crystals the apparatus used should be free of silicon, so that no glass but, if possible, a synthetic substance or if possible, a metal is used.

After the vapor deposition of the target plate 15 of the camera tube described with reference to FIG. 1, the free surface remote from the signal electrode 13 is exposed by means of a gas discharge in an oxygen atmosphere to an oxygen ion bombardment as is known for a target plate consisting substantially only of lead monoxide. This bombardment serves for incorporating an excess quantity of oxygen in the surface of the target plate in order to form a barrier for the electrons absorbed by the surface from the scanning electron beam 16.

The invention is explained in the foregoing with reference to the embodiment of a camera tube. However, the invention may also be applied to a photoconductive cell in which photoconductive layer containing mixed crystals of lead monoxide and tin monoxide is provided with two or more electrode, for example, formed by interdigital comb-shaped electrodes on one side of the layer or by planar electrodes one on each side of the layer. 1f the use of such a photoconductive cell allows cooling (resulting in a lower dark current), the tin monoxide content in the mixed crystals may, if necessary, exceed 25 mol percent.

What is claimed is:

1. A photoconductive device comprising an evacuated envelope having a transparent window portion, a support within said envelope, a photoconductive layer within said envelope on said support having at least one surface visible through said window portion, said photoconductive layer consisting essentially of lead monoxide and mixed crystals of lead monoxide and tin oxide, the tin oxide content of said mixed crystals being between 0.1 and mol percent, and positive and negative current supply means to said layer defining a direction of current flow through said layer..

2. A photoconductive device as defined in claim 1 in which said support is the window, and one of said current supply means is transparent and is positioned between said window and said layer, the other of said current supply means being an electron beam which is generated by an electron gun spaced from the photoconductive layer on the side opposite that facing the window, said device further including means for deflecting said electron beam to scan the surface of said photoconductive layer therewith.

3. A device as claimed in claim 2 characterized in that the photoconductive layer portion adjacent the support mainly consists of said mixed crystals and the layer portion adjacent the surface struck by the electron beam consists mainly of tetragonal lead monoxide.

4. A device as claimed in claim 1, characterized in that the tin monoxide content lies between 0.1 and 10 mol percent.

5. A device as claimed in claim 1 wherein the photoconductive layer consists substantially completely of the said mixed crystals.

Patent No. 3 610 ,987

Inventor(s) Dated Qctgher 5 122].

WIM KWESTROO ET AL It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2,

Column Column Column Column Column Column 6,

Signed and (SEAL) L Attest:

EDWARD M.FLETCHER,JR.

Attesting Officer line 1 change "10 to read -l0 line 7 change "10 to read line 19 change to read -u.

line 32 same as above.

line 59 change "10 to read -lO line 2, delete "characterized in that" and insert wherein.

line 7, delete "characterized in that" and insert wherein.

sealed this 20th day of November 1973.

RENE D. TEGTMEYER Acting Commissioner of Patents 

2. A photoconductive device as defined in claim 1 in which said support is the window, and one of said current supply means is transparent and is positioned between said window and said layer, the other of said current supply means being an electron beam which is generated by an electron gun spaced from the photoconductive layer on the side opposite that facing the window, said device further including means for deflecting said electron beam to scan the surface of said photoconductive layer therewith.
 3. A device as claimed in claim 2 characterized in that the photoconductive layer portion adjacent the support mainly consists of said mixed crystals and the layer portion adjacent the surface struck by the electron beam consists mainly of tetragonal lead monoxide.
 4. A device as claimed in claim 1, characterized in that the tin monoxide content lies between 0.1 and 10 mol percent.
 5. A device as claimed in claim 1 wherein the photoconductive layer consists substantially completely of the said mixed crystals. 